Cryoelectric switching trees



Jan. 18, 1966 R. w. AHRONS 3,230,391

CRYOELECTRIC SWITCHING TREES Filed Dec. 10, 1962 2 Sheets-Sheet 1 l6 '8o M2 Ac, III/l l4 i 4| PRIOR ART [/0 F| s.I 4o

DRIVE CURRENT A SOURCE 0/! 0/0 LEGEND m TIN [:1 LEAD FIG 2 l o o o 22,FF2|\FF 2 FF PRlOR ART 34 35 36 7o 1 ooo TIN LEAD g1 00' 80%} J PRIOR ARTOlO 4; 7i Fm 4 4 RT A 8l loo 1 as i 82 J I 76 no 4 9o 84 CURRENT JLL |uIZ SOURCE ''.1" 1 I I F l o o l 0 2 l 5 2 FF 2 FF 2 FF INVENTOR ard WAhrons ATTORNEY Jan. 18, 1966 R. w. AHRONS CRYOELECTRIC SWITCHING TREES2 Sheets-Sheet 2 Filed Dec. 10, 1962 mm vm momDom .PZMEEDO INVENTOR Rhard WAhrons u U n g ozwwmj A TTORNE Y United States Patent 3,230,391CRYOELECTRIC SWITCHING TREES Richard W. Ahrons, Somerville, N.J.,assignor to Radio Corporation of America, a corporation of DelawareFiled Dec. 10, 1962, Ser. No. 243,427 11 Claims. (Cl. 30788.5)

This invention relates to new and improved cryoelectric switching trees.

A desired path may be selected through a cryoelectric switching tree byplacing impedances in series with all except the desired path. Forexample, in the case of cryotron trees, the gate electrodes of cryotronsin the nonselected paths are made normal (made to exhibit a finite valueof resistance) and the selected path remains superconducting. A drivecurrent applied to the tree steers into the superconductive path inpreference to the other paths because it is the path of lowest (zero)resistance.

In prior art ciyoelectric switching trees, the resistance of eachcryotron is the same. The present inventor has discovered that improvedperformance can be obtained by employing impedances of different valuesin different paths through a selection tree. The impedances effectingcourse selection (as, for example, selecting 64 paths out of 128) aremade to exhibit a substantially higher value of impedance than theimpedances effecting fine selection (as, for example, the impedanceswhich effect the selection of one path out of two). The invention isapplicable both to resistive (cryotron) switching trees and to inductiveswitching trees. In the former case, improved operating speed isobtained by following the teachings of this invention. In the lattertype of trees, improved current distribution is obtained with thearrangement of the present invention.

The invention is discussed in greater detail below and is illustrated inthe following drawings of which:

FIG. 1 is a schematic showing of a prior art cryotron;

FIG. 2 is a schematic drawing of a prior art cry otron selection tree;

FIG. 3 is a schematic drawing of a cryotron selection tree according tothe present invention;

FIG. 4 is a schematic showing of the prior art inductive switch; and

FIG. 5 is a schematic showing of an inductive switching tree inaccordance with the present invention.

In the discussion which follows, an environment is assumed in whichsuperconductivity is possible. For example, the temperature is assumedto be only a few degrees Kelvin.

The cryotron of FIG. 1 includes a gate electrode formed of a softsuperconductor such as tin and a control electrode 12 formed of a hardsuperconductor such as lead. Both electrodes may be in the form of thinfilms which are insulated from one another. The cryotron is located overa ground plane 14 which is insulated from the electrodes 16 and 12.

In the operation of the cryotron, the gate electrode 10 may initially bein the superconducting state. In this condition, a drive current appliedto input terminal 16 of insufficient magnitude to cause gate 10 toassume a normal (resistive) state sees a low impedance path through thegate 10. If, however, a control pulse is applied to terminal 18, themagnetic field produced by the control pulse passing through the controlelectrode is of sufiicient magnitude to drive the gate from itssuperconducting to its normal state. When this occurs, the gate presentsa finite resistance to the drive current applied to terminal 16.

A prior art selection tree employing a cryotron is shown in FIG. 2.While in practice there may be many more than 8 paths for a drivecurrent, for the sake of drawing simplicity, only 8 such paths areshown. These paths Patented Jan. 18, 1966 may be, for example, the X orY drive lines for a superconductor memory (not shown). The selectiontree also has a ground plane which is not shown.

The selection tree of FIG. 2 includes 14 cryotrons 20-33, respectively.The desired one of the 8 paths for the drive current is selected byselection currents applied to certain ones of these cryotrons. Thesecurrents may be applied by the flip-flops 34, 35, 36.

In operation, assume that the 1 output terminals of the three flip-flopsare active and the 0 output terminals are inactive (at groundpotential). This causes the gate elec trodes of cryotrons 20, 23, 25,33, 31, 29 and 27 to become resistive. Accordingly, the only path of the8 paths which remains superconducting in its entirety is the pathlea-ding to 111 since the gate electrodes of cryotrons 21, 22 and 26 allremain in the superconducting state. If, during the time the flip-flopsare in the states indicated, a drive current pulse 37 from currentsource is applied to the convergent end 38 of the tree, the pulse willsteer into the superconducting path, since it has zero resistance, inpreference to the other paths. This is ind-icated schematically by thedashed arrow 42.

While the above is, in a rough way, how the cryotron selection treeoperates, additional factors must be considered in determining the speedcapability of the tree. The speed is influenced by the time required forthe selection currents (the currents provided by the flip-flops) todrive the cryotrons in series with the unselected current paths normal,and the time required for the drive current to follow the selected (thesuperconducting) path after the cryotrons have been driven normal. Itmay be assumed for the purposes of this discussion that the timerequired to drive the cryotrons normal is so short that it may beneglected. If, when the cryotron in the undesired paths are normal, thedrive current pulse 37 is applied to the convergent end 38 of the tree,it secs 8 different paths in parallel, each including an inductivecomponent and some including also a resistive component. As the pathsare substantially identical (from a physical viewpoint), the value ofinductance in each path is substantially the same.

When the drive current pulse from source 40 is initially applied, itshigher frequency components, that is, those components which togetherprovide the steep leading edge of the pulse, see mainly the inductanceof each path. Therefore, there arrives instantaneously at each of the 8paths (legended 000 through 111) a current having an amplitudeone-eighth that of the drive current pulse. Thereafter, due to theresistance present in the non-selected paths, the current applied tothese nonselected paths decays (see waveform 41), and the drive currentpulse steers into the selected path.

There is generally a considerable time At (see waveform 39) between thetime t at which the leading edge of drive current pulse occurs and thetime t when substantially the full drive current is delivered to theselected superconducting path. As the selection tree becomes larger, itcan be shown that At increases. The equations demonstrating this arecomplex and are not given here. However, the calculated delay At for a128 path cryotron switching tree similar to the tree of FIG. 2 is theorder of 5 microseconds.

In the present invention, as applied to cryotron switch ing trees, thetime At defined above is reduced substantially. In the case of 128 pathtrees, for example, the time At required by the tree of the invention(FIG. 3) is roughly /2 the time At required for a tree like the one ofFIG. 2. The time reduction increases as the tree becomes larger. Thereduction is achieved by essentially increasing the resistance in serieswith the nonselected paths through the tree. The increase in resistancecauses the current present in each non-selected path due to the initialinductive current division more quick- 1y to decay and consequently thedrive .current more quickly to steer out of each non-selected path andinto the selected superconductor path. Again, the equations whichdemonstrate this are quite complex. However, from a qualitativeviewpoint, one can consider that increasing the resistance R in serieswith a non-selected path causes a substantial decrease in the L/R timeconstant associated with that path, and this essentially speeds up theremoval of current from that path.

While it would be possible to improve the operating speed of a cryotronswitching tree by increasing uniformly the resistance of all gateelectrodes, this is found not to be practical. Increasing the resistanceof all gates can be achieved only by making the tree geometry morecomplex. For example, the tree geometry may require that the controlelectrodes not follow straight lines. Further, it may require that thespacing between the paths be increased because of the increasedcomplexity of the switching elements. In either case, intricate maskingwould be required and, moverover, the increased control lead lengthsand, in some cases, the zig-zag path they would be required to follow,would increase the time required for the selection pulses to reach thecryotrons in the tree. As is discussed in more detail later, the tree ofthe present invention is actually sirnplier to construct than the priorart tree of FIG. 2, as well as being faster than the tree of FIG. 2.

A 16 path cryotron switching tree according to the present invention isshown in FIG. 3. The tree includes a first group 50 of 15 cryotrons atthe left side of the tree and a second group 52 of 15 cryotrons at theright side of the tree. The group 52 of cryotrons is essentially aninverted mirror image of the group 50 of cryotrons. A ground plane (notshown) may be placed beneath the tree. The control electrodes for thecryotrons, in practice, are as wide as the gate electrodes and arealigned with the gate electrodes, however, for the sake of drawingsimplicity, the control electrodes are shown as single lines. Thecontrol electrodes are made of a hard superconductor such as lead andthe gate electrodes are made of a soft superconductor such as tin.

The drive current pulse source is shown at 54 and is connected to theinput end 56 of the tree. The control electrodes for the variouscryotrons are connected to the 1 and output terminals of the fourselection flip-flops 5861. The latter supply the selection currents.

It may be observed that the cryotrons employed are in-line cryotronsrather than crossed-field cryotrons. An in-line cryotron is one in whichthe control electrode is arranged parallel to the gate electrode,whereas a crossed-field cryotron is one in which the control electrodeextends at right angles to the gate electrode. It may also be observedthat the gate electrodes have a length which is related to thecoarseness of the selection step. The coarser the selection, the greaterthe gate electrode length, and therefore, the larger the resistanceintroduced. In the tree illustrated, the 2 flip-- flop 61 produces thecoarsest selection ste-p since the cryotrons it controls eliminate 8 ofthe 16 possible paths. Of the remaining 8 paths, the 2 flip-flop 60'eliminates 4, and so on.

Another feature of the cryotron selection tree of FIG. 3 is that thegate electrodes corresponding to a given selection bit are aligned. Thismakes the masking required for vacuum depositing the gate electrodesrelatively simple, and also makes the masking required for the controleleqtrodes relatively simple as they have a straight geometry.

The configuration of the paths through the tree of FIG. 3 is alsoadvantageous for a reason which is not so obvious. In practice, when onewishes to vacuumdeposit two lines which extend at right angles to oneanother as, for example, the lines 57 and 59 of the tree of FIG. 2, itis necessary to do this in two steps,

using a separate mask for each step, even though the two lines actuallylie in the same plane. The difiicutly with using a single mask is thatthe inner corner of the opening in the mask is unsupported, and,especially when the lines such as 57 and 59 are long, may vibrate ormove during the vacuum deposition process. This, of course, results infaulty patterns. With the arrangement of FIG. 3, all of the pathsthrough the tree, that is, all of the portions of the tree made of lead,are straight lines which extend horizontally. Thus, all of these pathscan be laid down through a single mask. In a similar manner, all of thegate electrodes (tin) lie on vertical lines and can be laid down througha single mask.

The operation of the system for FIG. 3 may perhaps be better understoodby specific example. Assume that it is desired to select the line 1101.This corresponds to active 1, l, 0, 1 output terminals of the 2 2 2 and2 flip-flops, respectively. The active 1 terminal of the 2 flip-flopcauses the gate electrode of the cryotron 62 to go normal. Thiseliminates paths 0111 through 0000. The active 1 terminal of the 2flip-flop causes the gate electrodes of cryotrons 64 and 66 to gonormal. The normal gate 66 eliminates the four paths 1011 through 1000.The active 0 terminal of 2 flip-flop causes the gate electrodes ofcryotrons 68, 70, 72 and 74 to go normal. The normal gate 74 eliminatespaths 1111 and 1110. The active 1 terminal of the 2 flip-flop causes thegate electrode of cryotron 76 and of all cryotrons aligned with cryotron76 to go normal. The normal gate 76 eliminates path 1100. Therefore, theonly path which remains superconducting is the path through line 1101,as indicated by dashed line 78.

In the operation of the selection tree of FIG. 3, the drive currentpulse applied to input lead 56 initially divides equally among the 16paths. It then decays from all of the non-selected paths and steerssubstantially entirely into the path 1101. The decay is greatlyaccelerated in the tree of the invention because of the increasedresistance in the non-selected paths. For example, the last 8 pathsinclude the very long gate electrode of the cryotron 62 (some of these 8paths also include other normal gate electrodes) and this electrode hasa relatively large resistance. In a similar manner, the paths 1011,1010, 1001 and 1000 have in series the relatively long gate electrode ofthe cryotron 66 and so on.

A prior art inductive switching element is shown in FIG. 4. It includesa control element formed of a soft superconductor such as tin and acontrolled element 72 which is preferably formed of a hardsuperconductor such as lead. When the control element 70 is in itssuperconducting state, the inductance exhibited by the controlledelement 72 is relatively low due to the shielding elfect of the element70. However, when the penetration of depth A of the element 70 isgreatly increased as, for example, by driving element 70 into itsintermediate or normal state, the inductance of the controlled element72 very greatly increases. While not shown, the controlled element 70may have a resistance of very low value in shunt therewith to permitelement 70 to be driven into the intermediate rather than the normalstate. (A more detailed discussion of the inductive switching element ofFIG. 4 may be found in Appl. Ser. No. 195,- 462, filed May 17, 1962, byR. A. Gange and assigned to the same assignee as the present invention.)

The inductive switching element of FIG. 4 may be substituted for thecryotron switching elements of FIG. 2 to provide an inductive switchingtree. In such a tree, all controlled elements are of the same length.When so employed, the controlled element '70 of the inductive switchingelements in all except a desired path through the tree are placed in theintermediate or normal state. Thus, all except the desired path'throughthe tree exhibit a relatively large value of inductance. Accordingly,when a drive current pulse is applied to the input end of the switchingtree, that pulse steers instantaneously into the desired path since itsinductance is substantially smaller than that of the remaining paths.Actually, the division of current is in accordance with the inductancesseen by the drive pulse. While this current division is instantaneous,there is no later redistribution of current as all paths are alwayssuperconductive. Therefore, if the inductance of a desired path is sayone-tenth that of the inductance of all other paths combined, thenninetenths of the drive current will flow into the desired path andremain in the desired path.

An improved inductive switching tree in accordance with the presentinvention is shown in FIG. 5. In this tree, the inductive switchingelements which control the coarse selection are made to have much longercontrolled and control electrodes than the inductive switching elementswhich control the fine selection. The inductance L exhibited by acontrolled element is a function of its length. The important advantageof this tree over the prior art inductive switching tree just describedis that the drive current distribution will be such that a greaterproportion of the drive current will flow into the desired path. This isbecause the effective value of inductance in shunt with the desired pathwill be much greater in the switching tree of FIG. 5 than in the priorart inductive switching tree already discussed and therefore lesscurrent will steer into the undesired shunt paths.

The operation of the switching tree of FIG. 5 may perhaps be betterunderstood by specific example. Assume that it is desired to select apath 010. This corresponds to the 0, 1, 0 output terminals active, ofthe 2 2 and 2 flip-flops, respectively. The active 0 terminal of the 2flip-flop causes the inductive switch 74 to be actuated. This places arelatively large value of inductance in series with paths 1%, 101, 110and 111. The active 1 terminal of the 2 flip-flop actuates inductiveswitches 76 and 78. Active switch 78 places a relatively large value ofinductance in series with paths 000 and 001. The active 0 terminal ofthe 2 flip-flop actuates inductive switches 8084. Active inductiveswitch 81 places a relatively large value of inductance in series withpath 011. The only path remaining which has only a relatively smallvalue of inductance associated with it is path 010. This is because thecontrol elements of the inductive switches 86, 87 and 88 remain in thesuperconductive state. If now a drive current pulse 90 is applied bycurrent source 92 to the input end 94 of the inductive switching tree,it will steer substantially instantaneously into the path 010. Theamount of current which divides into the remaining paths is greatlyminimized in view of the relatively large values of inducmnce in serieswith the remaining paths. For example, the large inductive switchingelement 74 exhibits a large value of inductance in series with paths 1%,101, 110 and 111.

What is claimed is:

1. In a cryoelectric switching tree in which there are a plurality ofpaths through the tree and which includes controllable individualimpedances which in one condition exhibits substantially higher value ofimpedance than in the other condition, essentially in series withdifferent groups of said paths, some said groups including more pathsthan others, the improvement comprising the impedances for the groupscontaining larger numbers of paths having larger values when in theirhigher impedance condition than the impedances for the groups containingsmaller numbers of paths when the latter impedances are in their higherimpedance condition.

2. In a cryoelectric switching tree in which there are a plurality ofpaths through the tree and which includes the respective gate electrodesof cryotrons essentially in series with different groups of said paths,some said groups including more paths than others, the improvementcomprising the gate electrodes of the cryotrons for the groupscontaining larger numbers of paths exhibiting a larger resistance, whendriven normal, than the gate electrodes of the cryotrons for the groupscontaining smaller numbers of paths, when the latter gate electrodes aredriven normal.

3. In a cryoelectric switching tree in which there are a plurality ofpaths through the tree and which includes the respective controlledelements of inductive switches which in on condition exhibits asubstantially larger value of inductance than when in the othercondition, essentially in series with different groups of said paths,some said groups including more paths than others, the improvementcomprising the controlled elements of the switches for the groupscontaining larger numbers of paths exhibiting a larger value ofinductance, when their respective control elements are driven out of thesuperconducting state, than the controlled elements of the inductiveswitches for the groups containing smaller numbers of paths, when thecontrol elements of the latter inductive switches are driven out oftheir superconducting state.

4. In a switching tree,

11 paths through the tree;

a switching element connected to one end of the paths essentially inseries with a first group containing 11/111 of the paths;

a second switching element connected to the other end of the pathsessentially in series with a second group containing n/ m of the paths,where none of the first group of paths is common to the second group ofpaths;

a third switching element connected to one end of the paths essentiallyin series with a sub group containing 11/ mp of the first group ofpaths;

and a fourth switching element connected to the other end of the pathsessentially in series with a sub group containing n/ mp of the secondgroup of paths, where n, n/m, and n/mp are all integers.

5. In a cryoelectric switching tree,

:1 paths through the tree;

a first cryotron the gate electrode of which is connected to one end ofthe paths essentially in series with a first group containing n/m of thepaths;

a second cryotron the gate electrode of which has a resistance, whennormal, which is substantially equal to that of the gate electrode ofthe first cryotron, said gate electrode of the second cryotron beingconnected to the other end of the paths essentially in series with asecond group containing 11/111 of the paths, where none of the firstgroup of paths is common to the second group of paths;

a third cryotron having a gate electrode which exhibits a lower value ofresistance, when normal, than the first cryotron, said gate electrode ofthe third cryotron being connected to said one end of the pathsessentially in series with a sub gnoup containing n/mp of the firstgroup of paths;

and a fourth cryotron having a gate electrode which has a resistance,when normal, which is substantially equal to that of the gate electrodeof the third cryotron, said gate electrode of the fourth cryotron beingconnected to the other end of the paths essentially in series with a subgroup containing n/mp of the second group of paths, where n, n/m, and11/ mp are all integers.

6. In a cryoelectric switching tree,

2 paths through the tree;

a first cryoelectric switching element connected to one end of the pathsessentially in series with a first group containing 2 /2 of the paths;

a second cryoelectric switching element connected to the other end ofthe paths essentially in series with a second group containing theremaining 2 2 of the paths, where none of the first group of paths iscommon to the second group of paths;

a third cryoelectric switching element connected to one end of the pathsessentially in series with a sub group containing 2 /4 of the firstgroup of paths and with the first cryoelectric switching element; and afourth cry oelectric switching element connected to the other end of thepaths essentially in series with a sub group containing 2 /4 of thesecond group of paths and with the second cryoelectric switchingelement, Where n in an integer.

7. In the cryoelectric switching tree of claim 6 said switchingelement's comprising the gate electrodes of inline cry otrons, and thefirst and second of said electrodes being longer than the third andfourth of said electrodes.

8. In a cryoelectric switching tree, the improvement comprisingthe pathsin one plane through the tree formed of a hard superconductor beingarranged parallel toone another and each having a straight linegeometry, and cryotron gate electrodes in series with the paths lying inthe same plane as and at an angle to the paths and arranged parallel toone another, said electrodes being formed of a soft superconductor andeach also having a straight line geometry.

9. In a cryoelectric switching tree, the improvement comprising thepaths in one plane through the tree formed of a hard superconductorbeing arranged parallel to one another and each being continuous andhaving 13. straight line geometry, and cryotron gate electrodesconnected to the paths lying in the same plane as and at an angle to thepaths and arranged parallel to one another, saidelectrodes each alsohaving a straight line geometry.

10. In a vacuum deposited cryoelectric switching tree, the improvementcomprising the paths in one plane through the tree formed of a hardsuperconductor being arranged parallel to one another and each beingcontinuous and having a straight line geometry, whereby all said pathsmay be deposited through a single mask, and cryotron gate electrodesconnected to the paths lying in the same plane as and at an angle to thepaths and arranged parallel to one another, said electrodes each alsohaving a straight line geometry, whereby all of said electrodes may bedeposited through a single mask.

11. In combination,

first and second superconductor lines;

a first superconductor gate element connected to one end of the firstline and a first superconductor bypass element which is joined to thefirst gate element, connected to the same end of the second line;

a second superconductor gate element connected to the other end of thesecond line and a second superconductor by-pass element which is joinedto the second gate element, connnected to the other end of the firstline;

means for applying a current to the first gate [and bypass elements; and

means for selectively driving one of the first and second gate elementsnormal.

References Cited by the Examiner UNITED STATES PATENTS 7/1962 Buckinghamet al. 30788.5 X

OTHER REFERENCES DAVID J. GALVIN, Primary Examiner.

ARTHUR GAUSS, JOHN W. HUCKERT, Examiners.

1. IN A CRYOELECTRIC SWITCHING TREE IN WHICH THERE ARE A PLURALITY OFPATHS THROUGH THE TREE AND WHICH INCLUDES CONTROLLABLE INDIVIDUALIMPEDANCES WHICH IN ONE CONDITION EXHIBITS SUBSTANTIALLY HIGHER VALUE OFIMPEDANCE THAN IN THE OTHER CONDITION, ESSENTIALLY IN SERIES WITHDIFFERENT GROUPS OF SAID PATHS, SOME SAID GROUPS INCLUDING MORE PATHSTHAN OTHERS, THE IMPROVEMENT COMPRISING THE IMPEDANCES FOR THE GROUPSCONTAINING LARGER NUMBERS OF PATHS HAVING LARGER VALUES WHEN IN THEIRHIGHER IMPEDANCE CONDITION THAN THE IMPEDANCCES FOR THE GROUPSCONTAINING SMALLER NUMBERS OF PATHS WHEN THE LATTER IMPEDANCES ARE INTHEIR HIGHER IMPEDANCE CONDITION.